[IR] Make getFirstNonPHI return null if the BB is empty

getFirstNonPHI's documentation states that it returns null if there is
no non-PHI instruction.  However, it instead returns a pointer to the
end iterator.  The implementation of getFirstNonPHI claims that
dereferencing the iterator will result in an assertion failure but this
doesn't occur.  Instead, machinery like getFirstInsertionPt will attempt
to isa<> this invalid memory which results in unpredictable behavior.
Instead, make getFirst* return null if no such instruction exists.

llvm-svn: 241570